This invention relates to a variable-speed power generating system including a variable-speed generator whose secondary winding is excited by an AC signal having a variable frequency, so that an electrical power output having a constant frequency can be always generated regardless of the rotation speed of the generator. More particularly, this invention relates to a variable-speed power generating system in which its variable-speed generator can stably operate even when the generator is disconnected from an associated electric power system due to occurrence of a trouble in the electric power system.
A conventional synchronous machine is commonly widely used as a generator for supplying electric power to an electric power system, and, in this case, the frequency of the AC voltage of the electric power system is always proportional to the rotation speed of the generator. On the other hand, when a variable or adjustable-speed generator which is basically similar to an induction machine is used to supply electric power to an electric power system, the rotation speed of the variable-speed generator can be freely selected independently of the output frequency while maintaining the output frequency to be equal to the system frequency. Thus, when such a variable-speed generator is combined with a driver which is, for example, a water turbine, the water turbine can be operated at a rotation speed at which the water turbine exhibits its highest turbine efficiency. Therefore, various researches and studies have been made hitherto on such a variable-speed power generating system. A variable-speed power generating system comprising the combination of a water turbine and a variable-speed generator is disclosed in, for example, Japanese patent application unexamined publication JP-A-55-56499. In the disclosed power generating system, the rotor side (the secondary side) of the variable-speed generator is excited by an alternating current of a variable frequency relating with the rotation speed of the rotor, so that an electric power output having a constant frequency can be always generated from the stator side of the generator without regard to the rotation speed of the rotor.
The structure of the disclosed variable-speed power generating system will be briefly described together with a main control system belonging thereto. The variable-speed generator is connected at its primary winding to an electric power system through a high-voltage side breaker, a main transformer and a low-voltage side breaker. This variable-speed generator has its primary and secondary windings disposed on the stator side and rotor side respectively and is coupled at its rotor shaft to the water turbine which drives the variable-speed generator. Further, the secondary winding of the variable-speed generator is connected through a frequency converter to the primary side of the variable-speed generator. Further, a permanent-magnet generator for detecting the rotation speed of the rotor and a phase detector for detecting the rotation phase of the rotor are coupled to the rotor shaft of the variable-speed generator.
In the variable-speed power generating system having a structure as described above, its output frequency f.sub.O is the sum of a frequency f.sub.N determined according to the rotation speed of the water turbine and a so-called slip frequency f.sub.S. One of principal or fundamental control units incorporated in the variable-speed power generating system is the speed governor governing the rotation speed of the water turbine which drives the variable-speed generator, and the speed governor controls the opening of the guide vanes of the water turbine, so that the water turbine can operate at a rotation speed (hence, the frequency f.sub.N) at which it exhibits its highest turbine efficiency. The aforementioned permanent-magnet generator is a rotation speed detector for controlling the rotation speed of the variable-speed generator by feedback control. Another fundamental control unit provided in the variable-speed power generating system is the aforementioned frequency converter generating an AC output for exciting the secondary winding of the variable-speed generator. The aforementioned phase detector detects the difference between the output frequency (=the system frequency) f.sub.O and the frequency f.sub.N determined according to the rotation speed of the water turbine, that is, the slip frequency f.sub.S, and the firing angle of thyristors constituting the frequency converter is controlled so as to cause the frequency of the converter output exciting the secondary winding of the variable-speed generator to be equal to the detected slip frequency f.sub.S.
According to the variable-speed power generating system having these two fundamental control units, the frequency converter applies its output having the slip frequency relating to the rotation speed of the rotor and the power system frequency to the secondary side of the variable-speed generator during trouble-free normal operation of the electric power system. (That is, the secondary winding of the variable-speed generator is excited by the converter output having a frequency equal to the slip frequency f.sub.S representing the difference between the frequencies at the primary side and the rotation speed respectively of the variable-speed generator during normal operation of the electric power system.) Therefore, the variable-speed power generating system can always generate electric power having the same frequency as the system frequency of the electric power system even when the rotation speed of the variable-speed generator deviates from the synchronous speed, that is, regardless of a slight change in the rotation speed.
Thus, the variable-speed power generating system can continue its power generating operation without any special problem. However, a problem which will be described below arises when a trouble, for example, grounding of the bus bar of the electric power system occurs accidentally. As soon as such a trouble has occurred, the high-voltage side breaker is released to disconnect the variable-speed generator from the electric power system. When the load is cut off, the frequency of the output voltage appearing at the primary side of the disconnected variable-speed generator is applied now as an input to the phase detector. Since such a frequency is not maintained constant any more, it is now impossible to apply the constant system frequency as the input to the phase detector. That is, before the load is cut off, the frequency of the output voltage generated from the primary winding of the variable-speed generator and applied as one of inputs to the phase detector is equal to the constant and unvariable frequency of the electric power system. Thus, even when the frequency f.sub.N determined according to the rotation speed of the variable-speed generator varies, the slip frequency f.sub.S only varies with the variation of the frequency f.sub.N, and the value of the frequency f.sub.O is maintained constant, as will be apparent from the relation f.sub.O (constant)=f.sub.N +f.sub.S. However, after the load is cut off, the value of f.sub.O is not maintained constant and unvariable any more in the electric power system, and f.sub.O does not mean the system frequency any more but merely means the frequency of the output generated from the primary winding of the variable-speed generator. Therefore, a variation of the frequency f.sub.N corresponding to the rotation speed of the variable-speed generator driven by the water turbine under such a condition results in corresponding variations of f.sub.S and f.sub.O, and the output frequency of the variable-speed generator is not maintained constant.
As is well known, an input and an output energy of such a variable-speed generator are normally balanced, and the mechanical energy generated by the rotation of the water turbine is equal to the electrical energy generated from the variable-speed generator. However, immediately after the load is cut off, the electrical energy generated from the variable-speed generator is nearly null, and, since almost all the mechanical energy generated by the rotation of the water turbine is consumed to increase the rotation speed of the rotor, the rotor is accelerated. Although the rotation speed of the accelerated rotor is finally converged to a fixed value by the function of the water-turbine speed governor, a transient speed variation occurs inevitably until the rotation speed of the rotor is settled at the fixed value. Thus, when one of the input signals to the phase detector is derived from the output side of the variable-speed generator, the slip frequency f.sub.S and the output frequency f.sub.O of the variable-speed generator tend to become unstable under the influence of the speed variation of the rotor. Especially, the slip frequency f.sub.S may deviate from its allowable frequency range, thereby preventing continuous excitation of the secondary winding, that is, preventing driving of the variable-speed generator with its secondary winding excitation under no load. This means that the secondary winding of the disconnected variable-speed generator must be once deenergized under no load condition and when it is to be pulled in the electric power system again, the secondary winding of the variable-speed generator must be excited again before it is connected to the electric power system again. Thus, in such a case, the variable-speed generator cannot be quickly pulled in the electric power system again. Further, because the output frequency f.sub.O of the generator cannot be fixed, a large length of time is required for finding the synchronizing conditions to pull the generator in the electric power system again.
Therefore, there has been a strong demand for a variable-speed power generating system including a variable-speed generator which can be driven in a no-load secondary-excitation mode in which its secondary winding is continuously excited under no load even when its load is cut off.